Re: RFC: Transparent Hugepage support

[Mike Travis <travis@sgi.com>]

Hi Andrea,

I will find some time soon to test out your patch on a
(relatively) huge machine and let you know the results.

The memory size on this machine:

	480,700,399,616 bytes of system memory tested OK

This translates to ~240k available 2Mb pages.

Thanks,
Mike

Andrea Arcangeli wrote:
> Hello Ingo, Andi, everyone,
> 
> On Thu, Oct 29, 2009 at 10:43:44AM +0100, Ingo Molnar wrote:
>> * Andi Kleen <andi@firstfloor.org> wrote:
>>
>>>> 1GB pages can't be handled by this code, and clearly it's not 
>>>> practical to hope 1G pages to materialize in the buddy (even if we
>>> That seems short sightened. You do this because 2MB pages give you x% 
>>> performance advantage, but then it's likely that 1GB pages will give 
>>> another y% improvement and why should people stop at the smaller 
>>> improvement?
>>>
>>> Ignoring the gigantic pages now would just mean that this would need 
>>> to be revised later again or that users still need to use hacks like 
>>> libhugetlbfs.
>> I've read the patch and have read through this discussion and you are 
>> missing the big point that it's best to do such things gradually - one 
>> step at a time.
>>
>> Just like we went from 2 level pagetables to 3 level pagetables, then to 
>> 4 level pagetables - and we might go to 5 level pagetables in the 
>> future. We didnt go from 2 level pagetables to 5 level page tables in 
>> one go, despite predictions clearly pointing out the exponentially 
>> increasing need for RAM.
> 
> I totally agree with your assessment.
> 
>> So your obsession with 1GB pages is misguided. If indeed transparent 
>> largepages give us real benefits we can extend it to do transparent 
>> gbpages as well - should we ever want to. There's nothing 'shortsighted' 
>> about being gradual - the change is already ambitious enough as-is, and 
>> brings very clear benefits to a difficult, decade-old problem no other 
>> person was able to address.
>>
>> In fact introducing transparent 2MBpages makes 1GB pages support 
>> _easier_ to merge: as at that point we'll already have a (finally..) 
>> successful hugetlb facility happility used by an increasing range of 
>> applications.
> 
> Agreed.
> 
>> Hugetlbfs's big problem was always that it wasnt transparent and hence 
>> wasnt gradual for applications. It was an opt-in and constituted an 
>> interface/ABI change - that is always a big barrier to app adoption.
>>
>> So i give Andrea's patch a very big thumbs up - i hope it gets reviewed 
>> in fine detail and added to -mm ASAP. Our lack of decent, automatic 
>> hugepage support is sticking out like a sore thumb and is hurting us in 
>> high-performance setups. If largepage support within Linux has a chance, 
>> this might be the way to do it.
> 
> Thanks a lot for your review!
> 
>> A small comment regarding the patch itself: i think it could be 
>> simplified further by eliminating CONFIG_TRANSPARENT_HUGEPAGE and by 
>> making it a natural feature of hugepage support. If the code is correct 
>> i cannot see any scenario under which i wouldnt want a hugepage enabled 
>> kernel i'm booting to not have transparent hugepage support as well.
> 
> The two reasons why I added a config option are:
> 
> 1) because it was easy enough, gcc is smart enough to eliminate the
> external calls so I didn't need to add ifdefs with the exception of
> returning 0 from pmd_trans_huge and pmd_trans_frozen. I only had to
> make the exports of huge_memory.c visible unconditionally so it doesn't
> warn, after that I don't need to build and link huge_memory.o.
> 
> 2) to avoid breaking build of archs not implementing pmd_trans_huge
> and that may never be able to take advantage of it
> 
> But we could move CONFIG_TRANSPARENT_HUGEPAGE to an arch define forced
> to Y on x86-64 and N on power.
> 
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